The invention relates generally to steam turbines and more specifically to exhaust hoods for efficiently diffusing steam to a condenser.
In the discharge of exhaust steam from an axial flow turbine, for example discharge of this exhaust steam to a condenser, it is desirable to provide as smooth a flow of steam as possible and to minimize energy losses from accumulation of vortices, turbulences and non-uniformity in such flow. Usually the exhaust from the turbine is directed into an exhaust hood and from there through a discharge opening in the hood in a direction essentially normal to the axis of the turbine into a condenser. It is desirable to achieve a smooth transition from axial flow at the exhaust of the turbine to radial flow in the exhaust hood and thence a smooth flow at the discharge opening of this hood into the condenser.
In the constructing of an effective exhaust hood for use with such an axial flow turbine it is desirable to avoid acceleration losses within any guide means employed therein and to achieve a relatively uniform flow distribution at the discharge opening of the exhaust hood for the most efficient conversion of energy in the turbine and effective supplying of exhaust steam to the condenser to which it is connected.
It is also desirable to achieve optimum efficiency at the last stage buckets of the turbine prior to exhaust from the turbine by achieving a relatively uniform circumferential and radial pressure distribution in the exit plane of the last stage buckets. Usually, attempts have been made to accomplish these results while employing a hood having as short an axial length as possible, so as to limit the axial size of the turbine train.
The prior art has employed, in the exhaust duct connected to the turbine, vanes, which have smoothly curved surfaces for effectively changing the axial flow of the steam from the turbine to the generally radial flow. For example of such an arrangement for converting the axial flow of the exhaust from the turbine to radial flow is shown in U.S. Pat. No. 3,552,877 by Christ et al. Further developments in prior art exhaust hoods for axial flow turbines, such as U.S. Pat. No. 4,013,378 by Herzog, have incorporated multiple sets of vanes for further smoothing flow. The exhaust hood includes a first set of guide vanes arranged in an exhaust duct connected to the turbine adjacent the last stage buckets thereof. These vanes are curved to provide a relatively smooth transition of steam flow from an axial direction to a generally radial direction. A guide ring circumferentially surrounds the first set of guide vanes and a plurality of secondary vanes are circumferentially spaced around this guide ring. Steam, which is discharged radially from the first set of vanes to the secondary vanes, is directed by the secondary vanes to the discharge opening of the exhaust hood. The secondary vanes are substantially equally spaced around the guide ring and are curved, at different angles to effect different angles of discharge of steam from these vanes. The angles of discharge are chosen so as to direct the steam toward the discharge opening of the exhaust hood in a manner achieving substantially uniform flow distribution across the exit plane of the last stage buckets and across the plane of the discharge opening. However, while such vanes may be optimized for one set of flow conditions, they may operate with significantly less effectiveness at other flows.
Diffusers, for example, are commonly employed in steam turbines. Effective diffusers can improve turbine efficiency and output. Unfortunately, the complicated flow patterns existing in such turbines as well as the design problems caused by space limitations make fully effective diffusers almost impossible to design. A frequent result is flow separation that fully or partially destroys the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. For downward exhaust hoods used with axial steam turbines, the loss from the diffuser discharge to the exhaust hood discharge varies from top to bottom. At the top, much of the flow must be turned 180 degrees to place it over the diffuser and inner casing, then turned downward. Pressure at the top is thus higher than at the sides, which are in turn higher than at the bottom.
FIG. 1 illustrates a perspective partial cutaway of a double flow steam turbine a portion of a steam turbine. The steam turbine, generally designated 10, includes a rotor 12 mounting a plurality of turbine buckets 14. An inner turbine casing 16 is also illustrated mounting a plurality of diaphragms 18. A centrally disposed generally radial steam inlet 20 applies steam to each of the turbine buckets and stator blades on opposite axial sides of the turbine to drive the rotor. The stator vanes of the diaphragms 18 and the axially adjacent buckets 14 form the various stages of the turbine forming a flow path and it will be appreciated that the steam is exhausted from the final stage of the turbine for flow into a condenser beneath (not shown).
Also illustrated is an outer exhaust hood 21, which surrounds and supports the inner casing of the turbine as well as other parts such as the bearings. The turbine includes steam guides (not shown) for guiding the steam exhausting from the turbine into an outlet 26 for flow to one or more condensers. With the use of an exhaust hood supporting the turbine, bearings and ancillary parts, the exhaust steam path is tortuous and subject to pressure losses with consequent reduction in performance and efficiency. A plurality of support structures may be provided within the exhaust hood. 21 to brace the exhaust hood and to assist in guiding the steam exhaust flow. An exemplary support structure 30 is situated to receive and direct the steam exhaust flow 35 from the steam turbine 10. The diffusion of the steam is restricted to the volume in the exhaust hood 21.
The exhaust hood 21 includes an upper hood 22 and a lower hood 23. The upper and lower hoods are joined along a horizontal seating surface 33. An upper part of the lower hood 23 is reinforced with support members 34 providing a support frame 36. The weight borne by the support frame 36 is transferred at support ledge 27 to a foundation 40.
FIG. 2 illustrates a schematic elevation view of a prior an exhaust hood for the double flow steam turbine 10 including an exhaust flow path 35. The steam turbine LP section consists of an inlet domain 20, turbine stages (nozzles 18 and buckets 14) and an exhaust hood 22 with diffuser 25. One of the main functions of the exhaust hood is to recover the static pressure and guide the exhaust steam flow 35 from last stage buckets 15 to the condenser steam outlet 26 to the condenser (not shown) underneath. The exhaust hood 21 includes the upper exhaust hood 22 and the lower exhaust hood 23. Flow from the last stage buckets 15, which could have very high swirl and high flow gradient in radial direction, enters the condenser through exhaust hood 21. Part of the flow 28 directly flows down to condenser through the lower exhaust hood 23 and the remaining flow 29 travels through upper exhaust hood 22. The flow in the upper exhaust hood 22 is directed by flow guide 32 and begins to turn 180 degrees from a vertically upward direction to downward direction over the inner casing 16 to reach the condenser. This results in strong vortex formation 38 behind the steam guide 24 in upper exhaust hood and minimizes the effective flow area between the steam guide and outer wall of the hood, thereby increasing losses in the steam path as well. This phenomena decreases the flow diffusion in upper half of exhaust hood, results in degradation of exhaust hood performance, which has direct impact on the last stage bucket performance.
Accordingly, it would be desirable to eliminate vortex flow in the upper exhaust hood and provide improved flow patterns and diffusion performance, particularly in the upper exhaust hood.